1. Field of the Invention
The present invention relates to a method for the production of polarization holograms, an apparatus for the production of polarization holograms and the use of the polarization holograms according to the invention as data stores, security features or diffractive optical elements for performing conventional optical functions.
2. Description of Related Art
A computer-generated hologram (CGH) is a digital hologram. The structures of the CGH are calculated with the use of special algorithms, which simulate the holographic recording process in the computer, on a virtual sampling grating. These structures are then transferred to a material carrier, for example by lithographic methods.
In contrast to CGH, in the classical, analogue recording methods the holographic structures are produced by interference, i.e. phase-constant superposition of laser beams.
An advantage of CGH is that it is possible to produce holograms of non-real, computer-calculated objects or object images which are present as a mathematical description. The use of CGH thus increases the flexibility in the production of diffraction structures compared with the analogue approach. Furthermore, the design of a writing device for digital holograms is more robust than that for analogue holograms since it is not necessary to take into account a time-constant phase coupling of at least two laser beams.
Computer-generated holograms are widely used, for example for interferometric shape testing of workpieces, for the production of security elements for protection from forgery and as diffractive optical elements (DOE) for performing conventional optical functions (lenses, prisms).
Methods for producing CGH and the use thereof as security elements are part of the prior art and are described, for example, in DE 19 926 698 A1. These are phase or amplitude holograms.
Polarization holograms are a special form of holograms which have been produced to date by analogue techniques, i.e. by interference of laser beams. A pattern of polarization directions is produced by superposition of laser beams and is fixed in an optically anisotropic material.
Particularly in security elements, polarization holograms have some advantages over the conventional established phase or amplitude holograms. For example, a polarization hologram is sensitive to the direction of rotation of a circularly polarized read beam; a polarization hologram diffracts right and left circularly polarized light in different directions (P. Rochon, V. Dronyan, A. Natansohn: Polarization Holographic Gratings in Azopolymers for Detecting and Producing Circularly Polarized Light, submitted for publication in International Conference on Applications of Photonics Technology (SPIE), Technical Report No. 48, Office of Naval Research, Grant: N00014-93-1-0615, R&T Code: 3132081). A polarization hologram therefore provides greater protection from forgery.
A material which can store the polarization direction of a write beam is required for transferring a GCH to a medium for the production of a polarization hologram. This is not possible with the customary materials to which CGH are usually transferred. Furthermore, in the usual lithographic transfer of CGH to a material carrier, a permanent diffraction structure which cannot be modified is produced. The transfer of the CGH into a re-writable medium is substantially more flexible.
Both aspects are fulfilled by so-called photoaddressable polymers. Photoaddresable polymers are known (Polymers as Electrooptical and Photooptical Active Media, V. P. Shibaev (editor), Springer Verlag, New York 1995). These substances are characterized by the ability to form oriented birefringence on exposure to polarized light. The birefringence patterns written in can be made visible in polarized light. The examples of this type are the side group polymers according to U.S. Pat. No. 5,173,381 which contain azobenzene groups.
It is furthermore known that localized birefringence can be written into layers of these polymers at any desired point using polarized light, the preferential axis of which birefringence moves on rotation of the direction of polarization (K. Anderle, R. Birenheide, M. Eich, J. H. Wendorff, Makromol. Chem., Rapid Commun. 10, 477-483 (1989)).
EP0622789 A1 describes how the writing of photoaddressable polymers can be generally effected: Order states are generated in a polymer layer by the influence of actinic light. Preferably used light is linearly or circularly polarized light, the wavelength of which is in the region of the absorption band of the photoinducibly configuration-changing side groups. On incidence of polarized light along the surface normal of a film, the result is a preferential orientation in the plane of the film, which is uniform throughout the film in the case of linearly polarized light, whereas the preferential direction is periodically modified according to the electric vector of the excitation light along the surface normal with the use of circularly polarized light. Exposure to unpolarized light produces a preferential orientation perpendicular to the plane of the film.
EP 0 622 789 A1 furthermore states that photoaddressable polymers are particularly suitable for the processing of images and for information processing by means of holograms, the reproduction of which can be effected by illumination with a reference wave. In the laid-open application, a hologram is produced by superposing two phase-constant light sources in the storage material.
It is furthermore stated that, in the case of analogue storage, the values of the grey scale can be adjusted continuously and with positional resolution.
The production of polarization holograms by writing a digitized pattern of polarization directions is not described.
Polarization holograms which are written by an analogue write technique into photoaddressable polymers are part of the prior art. An apparatus with which polarization holograms can be written into photoaddressable polymers by superposition of two circularly polarized laser beams is described in the application WO 99/57719 A1 (page 10, line 1 to page 14, line 16). With the apparatus, however, it is not possible to write computer-generated polarization holograms directly into photoaddressable polymers.
DE 19 620 588 A1 describes a method differing from the abovementioned methods and intended for writing into photoaddressable polymers.
In a generative first process, a layer of a photoaddressable polymer is illuminated over the whole area with a light source for polarized light and anisotropic birefringence is thus produced. In a second step, the material thus prepared is illuminated with short light pulses and the anisotropy produced over the whole area is changed in localized regions. The writing of computer-generated polarization holograms is not described.